The present invention relates to a process for preparing an iridium oxide film.
A phenomenon wherein a substance in the form of a thin film brings about a visual change from one color to another color or changes between colored and bleached states through injection of ions having positive or negative charges is called "electrochromism". This phenomenon has been utilized, e.g., for an electrochromic device. Examples of known inorganic materials which color anodically by an oxidation process include metallic oxides such as iridium oxide, rhodium oxide, nickel oxide, and cobalt oxide. Among them, iridium oxide has advantages such as high response speed and excellent chemical stability and therefore is under the most energetic investigation.
Examples of the method of preparing an iridium oxide film known in the art include (1) an anodic oxidation method and (2) a reactive sputtering method. Method (1), i.e., the anodic oxidation method comprises forming an iridium film on a substrate by sputtering or vacuum deposition and anodizing the iridium film in a sulfuric acid solution to prepare an iridium oxide film.
Method (2), i.e., the reactive sputtering method comprises conducting sputtering in an O.sub.2 atmosphere or an O.sub.2 /Ar atmosphere through the use of metallic iridium as a target to prepare an iridium oxide film.
However, method (1) is disadvantageous in that when the surface area of the film is large, it is impossible to form a homogeneous iridium oxide film all over the surface, and has further disadvantages such as leaching of iridium into a sulfuric acid solution and the necessity of a long period of time for the formation of iridium oxide.
Method (2) had a drawback that since the optimum deposition rate for forming an iridium oxide film was as low as 10 .ANG./min, it took 1 hr or longer to form a thin film necessary for use as a display device, e.g., an iridium oxide film having a thickness of about 900 .ANG.. Further, besides the above-described methods (1) and (2), method (3) wherein an iridium film is heated to form an iridium oxide film can be also considered as theoretically possible. However, the iridium oxide film disadvantageously crystallizes upon being heated to 300.degree. C. or above to lose its electrochromic characteristics. On the other hand, no iridium oxide can be formed when iridium per se is heated at 300.degree. C. or below.
For these reasons, in order to improve method (3), a proposal has been made on a method wherein a composite film comprising metallic iridium and carbon is heated and oxidized at about 250.degree. C. to prepare iridium oxide (see Japanese Patent Publication No. 62-57707).
However, the iridium oxide film prepared by this method cannot be completely bleached by an electrochemical process and disadvantageously brings about the so-called "residual coloration" in a bleached state. This unfavorably lowered the quality as a display when the iridium oxide film was used as a display device.